1. Field Of The Invention
This invention relates to an audio signal time-duration converter for compressing and expanding the timing of an audio signal.
2. Description Of The Prior Art
When an audio signal recorded at normal speed in, say, the magnetic tape of a tape recorder is reproduced at a speed higher than the normal speed, the reproduction time is shortened while at the same time the frequency band of the voice is accordingly increased. The reproducing of the signal at a speed lower than the normal speed, on the other hand, causes the reproduction time to be expanded, reducing the frequency band of the voice. Therefore, in order to restore the frequency band of the signal to the normal audio frequency band, it is necessary to change the speaking speed, that is, the tempo without adversely affecting the pitch, timbre or the intelligibility of the voice.
The conventional device for conversion of an audio signal frequency band for similar purposes, as disclosed in U.S. Pat. No. 3,621,150, comprises a pair of shift registers in which input audio signals are alternately written at a predetermined sampling rate, while the contents of the registers not in a storage state are read at a rate different from the sampling rate thereby to effect the expansion and compression of the frequency band of the input audio signals. In such a device, two memory means are used as storage means. According to the present invention, by contrast, there is provided a random access memory means in which audio signals are written at a predetermined rate while the contents thereof are read at a different rate.
If a random access memory means is to be made most of, however, there are a number of problems to be overcome. One of the greatest problems is the fact that it is impossible to designate an address for a write operation and an address for a read operation at the same time. Instead, it is necessary to give priority to either the write or the read operation in designating an address. Further, this designation must be effected easily, accurately and reliably.
Also, in view of the fact that write clock pulses and/or read clock pulses for write and read operations respectively vary in their rate according to the reproducing speed of the magnetic tape, the relative changes in the rate of write and read clock pulses must preferably be accomplished easily.
Since the output of the random access memory means is produced both at the time of writing and reading, a read signal produced at the time of reading contains a write signal, that is, unneccessary data. This data should preferably be strobed and it is also preferable to regulate its time width.
In a conventional apparatus an analog-digital (A-D) converter, such as a converter of the pulse code modulation (PCM) type, is used to obtain digital signals written in the random access memory means. The apparatus according to the invention, however, employs a low-cost and simple delta modulator in place of the expensive and complex PCM converter. A digital-analog (D-A) converter is used to convert into an analog signal a digital signal read from the random access memory means. Comparison of the input audio signal to the delta modulator with the output audio signal of the D-A converter shows that the level of the output audio signal changes relative to the input audio signal depending on the changes in tape speed. The input and output level therefore must be adjusted for each change in input and output levels. The reduction in output or input level poses no problem as the gain of the amplifier is improved. When the level rises, however, the signal-to-noise ratio of the amplifier is deteriorated. Therefore, preferably, the levels of input audio signal and output audio signal should substantially remain unchanged against the variations in tape speed.
Also, the output of the delta modulator, the dynamic range of which is narrow, is deteriorated in signal-to-noise (S/N) ratio at an improper input level, thus making it necessary to maintain the input level almost constant.